1. Field
The present disclosure relates to casting prefabricated prestressed concrete products, such as slipform cast concrete products. More precisely the present disclosure relates to stressing of the prestressed reinforcement strands in the casting process.
2. Description of Related Art
Prefabricated concrete elements, such as hollow-core slabs and solid concrete slabs, are conventionally cast by slipform casting on long casting beds as a continuous casting process. The length of said continuous casting process is defined either on the basis of the total length of the elements to be cast, or on the basis of the maximum length of the casting bed. The length of casting beds used in slipform casting can be from 50-60 m up to 150-200 m, depending on the size of the element plant. When a slipform casting equipment has cast a continuous slab on a casting bed, the cast concrete slab is allowed to be cured on the casting bed. After the concrete mass has cured, the uniform cast element is cut, generally by sawing, into pieces with predetermined lengths on the basis of the design characteristics of the ready-made elements, and the cut concrete elements are lifted off the casting bed to storage, to wait for transportation to their appointed targets of usage.
Generally concrete elements cast by slipform casting are prestressed, i.e. they are provided with prestressed reinforcement strands. These reinforcement strands are prestressed by pulling the strands to a predefined stress before starting of the actual slipform casting with a suitable slipform casting machine.
The stressing of reinforcement strands may be carried out strand by strand, or in a bundle, where all of the required reinforcement strands are connected to a single strand pulling plate after which the plate is moved a predetermined distance with a bundle stressing device in order to achieve the required stressing of the reinforcement strands.
The problem with this bundle-type stressing of the reinforcement strands is whether proper stressing is achieved to all of the strands in bundle. Correct stressing of the reinforcement strands greatly affects the properties of the cast concrete slab, especially in view of the load bearing capacity of the slab.